Because valve seats used in internal combustion engines are exposed to a combustion gas at high temperatures and high pressure, and repeatedly undergo high shock and sliding by the vertical and rotational motions of valves, heat resistance and wear resistance are generally needed. Further, because of recent trend of lower fuel consumption, lower emission and higher power of internal combustion engines such as automobile engines, etc., combustion has become more controlled. Particularly, the use of a clean fuel such as CNG, LPG, etc. for reducing environmental load results in a higher combustion temperature, giving an increased thermal and mechanical load to valve seats. In addition, because combustion occurs in a higher-oxygen-concentration atmosphere than ever in a lean-burn combustion technology developed to improve fuel efficiency, valve seats are required to have larger heat resistance and high-temperature strength, as well as higher oxidation resistance.
Conventional valve seats used in internal combustion engines using a gas fuel are exposed to higher temperatures than in those using a liquid fuel, with no combustion products accumulated on sliding surfaces of valves, so that their sliding portions undergo metal contact, resulting in drastically increased wear.
JP 11-12697 A discloses a sintered alloy for valve seats capable of keeping excellent wear resistance and low attackability to mating members, even under conditions of easily causing metal contact between valve seats and valves, like when used in gas fuel engines, the sintered alloy comprising as matrix components at least 0.5-1.5% of C, and 0.5-10% of Cr and/or V, the balance being Fe, and containing 26-50% by weight of hard cobalt-based particles. Also, JP 2002-285293 A discloses a sintered alloy for valve seats exhibiting excellent high-temperature wear resistance in high-load engines such as CNG engines, heavy-duty diesel engines, etc., the sintered alloy having a composition comprising by mass 12.7-35.3% of Co, 5.4-16.2% of Mo, 1.8-6% of Cr, 0.02-0.24% of V, 0.4-1.5% of Si, 0.6-1.2% of C, and 0.01-1.8% of Ni, the balance being Fe and inevitable impurities, and a metal structure in which hard phases mainly composed of Mo silicide surrounded by Co diffusion phases are dispersed in bainite with or without sorbite. Further, JP 2006-299404 A discloses a sintered iron-based alloy usable for valve seats for gas fuel engines, which has a matrix composition comprising by mass 0.3-1.5% of C, and 1-20% in total of one or more selected from the group consisting of Ni, Co, Mo, Cr and V, the balance being Fe and inevitable impurities; and contains one or more intermetallic compounds comprising Fe, Mo and Si as main components, and intermetallic compounds comprising Ni, Mo and Si as main components, as hard particles; the sintered iron-based alloy containing 10-60% by mass of hard particles having Vickers hardness Hv0.1 of 500-1200, and having a density of 6.7 g/cm3 or more and radial crushing strength of 350 MPa or more.
Any of the sintered alloys disclosed in JP 11-12697 A, JP 2002-285293 A and JP 2006-299404 A has improved wear resistance and heat resistance by containing Co in a matrix and/or hard particles. However, because Co hinders the formation of a dense oxide film having excellent adhesion, an oxide film is hardly formed on the sliding surface particularly at as low temperatures as 250° C. or lower, resulting in insufficient wear resistance.
As a Co-free, sintered iron-based alloy, JP 4299042 B discloses a sintered iron-based alloy containing 3-20% by mass of hard particles dispersed in a matrix comprising by mass 3-12% of Ni, 3-12% of Mo, 0.1-3% of Nb, 0.5-5% of Cr, 0.6-4% of V, and 0.5-2% of C, the balance being Fe and inevitable impurities; the matrix comprising an iron matrix in which Ni, Mo, Cr, Nb and V are dissolved, and carbides of Mo, Cr, V and Nb and/or two or more intermetallic compounds of Mo, Cr, V and Nb as dispersed particles. It teaches third hard particles comprising 60-70% of Mo, and 0.1% or less of C, the balance being Fe, as Co-free hard particles. It teaches that a valve seat formed by this sintered iron-based alloy is suitable for gas engines subjected to larger mechanical and thermal load at high temperatures, particularly because both of fine Nb carbide and Nb dissolved in the prealloy increase high-temperature strength without a secondary treatment such as copper infiltration.
Pointing out that the third hard particles of JP 4299042 B comprising 60-70% of Mo, and 0.1% or less of C, the balance being Fe, do not have enough adhesion to the matrix, JP 4368245 B teaches that the adhesion is improved by hard particles comprising 60-70% of Mo, 0.3-1.0% of B, and 0.1% or less of C, the balance being Fe and inevitable impurities.
Though the Co-free, sintered iron-based alloys having excellent high-temperature strength, which are disclosed in JP 4299042 B and JP 4368245 B, are free of a problem that a dense oxide film having good adhesion cannot be formed, an oxide film is not sufficiently formed at low temperatures of 250° C. or lower, leaving room for improvement in the wear resistance of valve seats for gas engines.
Because a sliding surface of a valve seat coming into contact with a valve is machined when assembled in an engine head, good machinability is indispensable, so that the valve seat should have higher machinability.